[XCore] Fix handling of functions where only the LR is spilled.

Previously we only checked if the LR required saving if the frame size was
non zero. However because the caller reserves 1 word for the callee to use
that doesn't count towards our frame size it is possible for the LR to need
saving and for the frame size to be 0.

We didn't hit when the LR needed saving because of a function calls because
the 1 word of stack we must allocate for our callee means the frame size
is always non zero in this case. However we can hit this case if the LR is
clobbered in inline asm.